Rtc Holdup Calculation - Intel Quark SoC X1000 Design Manual

Figure 43. A Schottky Diode Circuit to Connect RTC External Battery
VCC3P3_S5
A standby power supply should be used in a mobile system to provide continuous
power to the RTC when available, which will significantly increase the RTC battery life
and thereby the RTC accuracy.
Using a rechargeable coin battery or a capacitor with a short projected discharged time
can increase the risk of:
• unusable anti-replay blobs
12.4.1

RTC Holdup Calculation

The example in
Super Cap, value 0.22uF/5.5V. The Panasonic EEC-SDHD224V is used in this example
circuit. The SDHD224V has a ESR of 75
The discharge time is calculated as follows;
T = -RC x ln(V/V0)
R: The RTC load = 3V/12uA
C: The Super Cap = 0.22F
Therefore RC = 55000
The discharge slope is based on a natural log, ln(V/V0) from 3.6V to 2V
V: 2V, minimum RTC operational voltage.
ln(V/V0) = ln (2/3.6) = -0.43825
Discharge time (T) = -55000 x -0.43825 = 24104 sec = ~6.7hours.
1. Estimated load at 12uA
®
Intel Quark™ SoC X1000
PDG
90
1.4V drop
0.5V drop
Schottky Diodes
R3
1K 
R4
3.6K 
C3
0.22F/5.5V
Vbatt
3V Lithium Battery
(Optional Use)
Figure 43 shows Super Capacitor, C3, this is a Panasonic SD Series
1
= 250000.
®
Intel Quark™ SoC X1000—RTC Design Guidelines
1uF
R2
240 
The ESR limits the charge
current and is negligible for
the RTC input current.
The charge time is ~3-4
minutes but the Hold up time
is ~6.7 hours.
VCCRTC
0.1uF
June 2014
Order Number: 330258-002US